1. Field of the Invention
The present invention relates generally to high pressure fuel pumps for supplying fuel to internal combustion engines. More particularly, the present invention relates to fuel pump cooling using parallel cooling fuel flow.
2. Description of the Related Art
Today's engine designers must meet the challenge of government mandated emissions criteria while striving to improve engine fuel efficiency. In rising to this challenge, designers create fuel systems that operate at higher pressures than systems of the past. As fuel pressures are increased to excess of 2600 bar, cooling and dilution become problematic with oil lubricated fuel pumps. Further, an increase in pressure leads to an increase in core temperature of the engine combustion area.
Fuel pumps typically include a pump plunger positioned in a bore of a fuel pump barrel and sized so as to permit reciprocating motion within the bore. Pump plungers are driven by a drive system located in a separate mechanical compartment and supplied with lubricating oil. Because the plunger diameter must necessarily be less than the bore diameter, fuel leakage in the resulting space can occur. The clearance gap between pump plunger and barrel is ideally minimized through precision matching of the barrel and plunger to reduce fuel leakage. An increase in barrel temperature, however, causes thermal expansion of the barrel material and therefore necessitates a looser fit between barrel and plunger to permit reciprocating plunger movement at elevated temperature. With a looser plunger/barrel fit, however, fuel is prone to escape from the fuel-pumping chamber and pass along the clearance space between plunger and barrel. This leakage fuel passes into the drive system mechanical compartment and contaminates engine lube oil, thus causing a reduction in oil viscosity and shortening oil life and effectiveness. Accordingly, what is needed is a fuel pump that can provide adequate pressurization to meet modern design standards yet employ a cooling system that effectively maintains fuel pump barrel temperatures for efficient mechanical operations.